The Mental Accounting Barrier to Micropayments   (link)

http://szabo.best.vwh.net/micropayments.html
psychological pressure regarding the value of the good or service purchased applied to the ratio between price and volume of product (i.e. you buy a lot, therefore the pressure is compounded and not feasible for the average consumer)
i think it's a crock (the concept itself is not, but a good implementation will avoid what this paper identifies as a key potential problem with the micropayment driven business model), but it's a good read anyways